1. Field of the Invention
The present invention relates generally to magnetic recording media, and more particularly to high density magnetic recording disks having textured surfaces.
Thin film magnetic recording disks generally comprise a disk substrate having a magnetic layer and a number of underlayers and overlayers deposited thereon. The nature and composition of each layer is selected to provide the desired magnetic recording characteristics, as generally recognized in the industry. An exemplary present day thin film disk is illustrated in FIG. 1 and comprises a non-magnetic disk substrate 10, typically composed of aluminum or an aluminum alloy. An amorphous nickel-phosphorus (NiP) layer 12 is formed over each surface of the disk substrate 10, typically by plating. The NiP layer is hard, and imparts rigidity to the aluminum substrate. A second underlayer in the form of a chromium ground layer 14 is formed over the NiP layer 12, typically by sputtering, and a magnetic layer 16 is formed over the ground layer 14. The magnetic layer 16 comprises a thin film of a ferromagnetic material, such as a magnetic oxide or magnetic alloy. Usually, a protective layer 18, such as carbon film, is formed over the magnetic layer 16, and a lubricating layer 20 is formed over the protective layer.
The presence of the NiP layer 12 and the chromium ground layer 14 have been found to improve the recording characteristics of the magnetic layer 16. In particular, a chromium ground layer formed over a NiP layer has been found to provide enhanced coercivity and reduced noise characteristics. Additionally, the NiP layer is often mechanically textured to create a roughened surface prior to formation of the chromium ground layer. This surface texturing has a substantial effect on the mechanical properties of the disk and its interaction with the recording transducer (read/write head), which typically "flies" over the disk surface on a cushion of air that is moved by the rotating disk. In particular, texturizing is highly beneficial to the magnetic recording system's ability to reliably withstand repeated starting and stopping of the disk, with its associated repeated contact between the read/write head and the disk's surface. The texturing may be circumferential, crosswise, or separated into start/stop and data zones, with the preferred geometry depending on the particular composition of the cobalt-containing magnetic layer, and on the specific disk drive design.
Such magnetic recording constructions have been very successful and allow for relatively high recording densities. As with all successes, however, it is desired to provide magnetic recording disks having even higher recording densities. To increase recording densities beyond those of known practical magnetic recording media, it would be beneficial to minimize any anisotropy of the magnetic recording layer within the structure of the magnetic recording media.
Work in connection with the present invention has shown that anisotropy (here referring to differing circumferential and radial magnetic recording characteristics) may deteriorate certain electrical and magnetic parameters of high density magnetic recording media, such as the pulse width, the overwrite, the non-linear transition shift, and the signal-to-noise ratio. Specifically, scratches in the disk structure prior to deposition of the magnetic layer are a recognized source of magnetic recording defects. These defects are worsened by magnetic layer anisotropy, as the film stress along the scratch will tend to align the preferred magnetic orientation of a subsequent anisotropic magnetic layer with the direction of the scratch. Where the scratch has a radial orientation and the recording media has a circumferential orientation, the scratch produces a localized radial orientation of the magnetics, which will cause disk magnetic defects and missing pulses. Clearly, such defects should be minimized. Therefore, it is desirable to provide isotropic media (media having substantially equivalent circumferential and radial recording characteristics) to optimize magnetic recording densities.
Unfortunately, the texture imposed on the NiP-coated substrate to provide acceptable mechanical characteristics and reliability can also induce substantial anisotropy in the final magnetic recording media. This is the same effect as the scratch-induced localized anisotropy, only it occurs along a texture line, generally with a circumferential orientation. This texture-induced anisotropy, which can result in coercivities in the circumferential direction being twice the radial coercivity or more, typically requires a tradeoff between mechanical and magnetic recording characteristics. In other words, the texture-induced anisotropy of present magnetic recording media structures requires some compromise of either reliability or recording density.
For these reasons, it would be desirable to provide an improved magnetic recording disk which allowed optimization of the texture for mechanical properties, and which avoided the texture-induced anisotropy of known magnetic recording media. It would be particularly desirable if such isotropic textured media maintained the enhanced magnetic properties which are provided by known media structures. It would further be desirable if such improved magnetic recording media were readily fabricated using existing thin film deposition and texturing equipment, and provided the enhanced magnetic properties of recent cobalt-containing magnetic layers.
2. Description of the Background Art
The magnetic properties and read/write characteristics of CoCrPt and CoCrTa magnetic layers disposed over ground layers of Cr or CrTi were described by Y. Shiroishi et al. in the Journal of Applied Physics, Vol. 73, No. 10, p. 5569 (May 1993). The CrTi underlayer was found to enhance the inplane coercive forces of these specific cobalt-containing magnetic layers. The magnetic recording characteristics of glass disks having pre-coat layers of Cr, Ta, W, or Zr were described by Hiroyuki Kataoka et al. in IEEE Transactions on Magnetics, Vol. 31, No. 6, p. 2734 (November 1995). In particular, the magnetic and recording characteristics of a CoCrPt magnetic layer disposed over a CrTi ground layer, in turn disposed over the pre-coated glass disks, were compared, with the Ta pre-coated glass disk being preferred due to its relatively large output signal and relatively low noise.
U.S. Pat. No. 5,166,006 describes a method for selectively chemically etching a substrate to form an isotropic texture of a desired density and roughness. Selective texturing of specific zones of the substrate are also described. U.S. Pat. No. 5,302,434 describes a contact recording media having an NiO film between an NiP-coated substrate and Cr or CrV layer to increase the coercivity of a subsequent cobalt alloy magnetic layer. U.S. Pat. Nos. 5,388,020, 4,698,251, 4,996,622, 5,478,622, and 4,939,614 describe textured recording media and methods for their production. U.S. Pat. No. 5,113,302 is generally relevant.